1. Field of the Invention
The present invention deals with powdered coating materials based on a mixture of polyesters and blocked polyisocyanates.
2. Description of the Prior Art
Pulverizable polyepoxides were the first bonding agents that found a great amount of interest as raw materials for powdered lacquers. It was only in 1965 that phenol-blocked polyisocyanates were described as new raw materials for powdered lacquers in DL-PS No. 55 820. Powdered mixtures of high melting point hydroxyl-group containing polyesters and polyisocyanates, whose isocyanate groups are blocked by phenol, can be applied by electrostatic spraying or by spraying on the substrate and can subsequently, be hardened into coatings by heating. The disadvantages of this process are caused by the splitting-off of phenol: strong bothersome odors, flowing problems and, especially, blister formation.
According to the experience of DT-AS No. 1,957,483, these disadvantages can be avoided if .epsilon.-caprolactam-blocked polyisocyanates are used instead of the previously mentioned phenol-blocked compounds.
However, for many areas of application, the reactivity of these .epsilon.-caprolactam-blocked, aliphatic diisocyanates and hydroxyl-containing polyester systems, as described in DT-AS No. 1,957,483, is insufficient, even in the presence of catalysts.